2. Field of the Invention
The present invention relates to a wavelength division multiplexing filter comprised of optical circuits formed on a planar substrate such as a Si substrate, and more particularly, to a wavelength division multiplexing (WDM) filter comprised of an optical circuit structure capable of improving a wavelength separation (crosstalk) characteristic.
2. Description of the Prior Art
The rapid and wide spread of Internet and e-mails is rapidly increasing the capacity of communication circuits principally in the U.S. A key technique for increasing the capacity is a dense wavelength division multiplexing (DWDM) system that allows signal lights of different wavelengths to simultaneously propagate through a single optical fiber in order to substantially increase the communication capacity. The number of wavelengths multiplxed by this DWDM system is being increased from 8 to 16 channels, and the transmission capacity is being increased in response to development of multimedia industries such as Internet.
The DWDM system typically splits a light into 8 or 16 wavelengths at intervals of 100 or 200 GHz or combines such wavelengths together. Thus, a high-performance wavelength filter is a key device. The wavelength filter is required to accurately align wavelengths and to have a low loss and a high performance.
Interference film filter type optical combining devices principally comprised of micro-optics have been used up to eight waves. An increase in the number of channels, however, has increased the costs of this interference film filter type, so that an increasing number of planar lightguide circuits (PLC) are now used for relevant applications. This trend is assumed to be more distinct as the number of wavelengths increases.
The most general PLC filters are devices comprised of silica-based-glass waveguides. Silica-based-glass waveguides enable devices using an interference effect to be precisely produced to construct high-performance filters. In particular, silica-based-glass waveguides commonly used as wavelength division multiplexing filters are arrayed waveguide gratings (AWG) each principally comprised of a plurality of input waveguides, a plurality of output waveguides, a pair of slab waveguides, and a waveguide array consisting of a large number of waveguides of different lengths.
FIG. 1 shows a basic configuration of a wavelength-multiplexing filter (a wavelength combiner/splitter) based on AWGs. In this figure, 1 is a Si substrate; 2 is input waveguides; 3 is output waveguides; 4a and 4b are slab waveguides; and 5 is a waveguide array. FIG. 2 shows a typical spectrum from this filter. Since the output waveguide of an AWG depends on the wavelength, wavelengths can be combined or split. Due to their low loss, high wavelength accuracy, and good crosstalk characteristic, AWGs comprised of silica-based-glass waveguides are used for DWDM systems.
However, as the number of wavelengths for wavelength multiplexing increases, higher performance is required of filters, and particularly, an improved crosstalk characteristic is required in order to separate a large number of wavelengths. For example, for an AWG comprised of silica-based glass waveguides and having 32 channels are required to maintain crosstalk at 30 dB or more. As described above, an increase in the number channels allows the features of PLC AWGs to be exhibited, whereby improvement of the crosstalk characteristic is important to the AWG.
Furthermore, if the DWDM has a small separated wavelength interval of 25 GHz or less, the AWG circuit is easily subjected to phase errors in the waveguides, thereby degrading the crosstalk characteristic. Since the number of channels normally increases with decreasing wavelength interval, the effect of crosstalk characteristic degradation becomes distinct. Thus, improvement of the crosstalk characteristic is essential in using for a WDM system, AWGs having a small wavelength interval and a large number of channels.
In order to improve the crosstalk characteristic, two filters are conventionally used. In a configuration with an AWG provided as a first filter, an external filter is connected in each output port of the AWG to improve the crosstalk characteristic.
FIG. 3 shows a general configuration of a conventional wavelength division multiplexing filter with its crosstalk characteristic improved using external filters. In this figure, 6 is a wavelength division multiplexing filter based on AWGs, 7 is an external filter, and 8a, 8b, 8c are optical fibers. The external filter 7 comprises an interference film filter and is structured so that the interference film filter is located between the optical fibers each having a SELFOC lens attached to its tip.
Parallel beams are emitted from the SELFOC fibers. Due to its characteristics including a low loss and an excellent crosstalk characteristic, the interference film filter is effective as an external filter in terms of optical characteristics.
As described above, although the conventional DWDM filters generally include external filters to improve the crosstalk characteristic, this method has the disadvantage of increased costs and size. Another problem of this method is that fiber routing is cumbersome.